(1) Field of the Invention
The present invention relates to an image forming apparatus that transfers toner images formed on an image carrier onto transfer objects.
(2) Related Art
An image forming apparatus such as a printer employing an electronic photography system has a configuration in which toner images are formed on an image carrier such as a photosensitive drum and intermediate transfer belt, and the toner images formed on the image carrier are transferred onto recording sheets such as sheets of paper.
According to this configuration, although it is ideal that all the toner images on the image carrier are transferred onto the recording sheets, part of toner particles actually sometimes remains, as residual toners, on the image carrier without being transferred. Also, there is a case where paper powders, which are generated due to contact with the sheets of paper, toner micropowders, which are generated by grinding toner particles, external additive particles, which are isolated from toner particles, and the like are deposited on the image carrier, in addition to residual toners.
If residual toners, paper powders, micro toner powders, external additive particles, and the like remain deposited on a surface of the image carrier, subsequent formation of toner images will be interfered with. For this reason, a cleaning unit is provided for cleaning the surface of the image carrier.
Such a cleaning unit has a typical configuration of scraping and removing residues on the image carrier by bringing a cleaning blade, a brush, or the like into contact with the image carrier which is rotating.
According to the configuration in which the cleaning blade, the brush, or the like is used, residual toners having a comparatively large size (in units of μm, for example) are easily removed. However, there is often a case where paper powders, external additive particles, and the like having a much smaller size (in units of, nm for example) than the residual toners cannot be completely removed from the surface of the image carrier. This is because the paper powders, the external additive particles, and the like are fine and accordingly are higher than the residual toners in terms of adhesive force (such as the Van der Waals force) relative to the surface of the image carrier.
If such micro paper powders, external additive particles, and the like remain deposited on the surface of the image carrier and then stick to the surface of the image carrier, a so-called filming easily occurs on the surface of the image carrier. In the case where filming occurs, difference is generated in transfer current between a region where the filming occurs and a region where no filming occurs on the surface of the image carrier due to difference in volume resistance therebetween. This causes non-uniform transfer, and results in non-uniform density in formed images.
Particularly in recent years, low-temperature fusing toners for fixing toners at a low temperature are used for the purpose of energy saving, and temperature inside the printer increases in accordance with the increase in printing speed. As a result, micro toner powders easily fuse due to the increase in the temperature inside the printer. Accordingly, paper powders, external additive particles, and the like tend to mix with the fused micro toner powders on the image carrier, and to be deposited on and stick to the image carrier. This tends to increase growth speed of a filming. Accordingly, early occurrence of filming easily results in formed image with un-uniform density. In view of this, improvement of cleaning properties is demanded.